AK Grandpa wrote:
OK, I have a nikon 40mm micro (macro) f2.8 . . . If I put extension tunes on it will my focusing distance be closer or farther. It focuses 1:1 at about 5 inches which is fine for flowers, but I'd like to be able to back off a bit for insects. Camera body is D5500 (DX) . . . Thanks . . .
Extension tubes will enable the lens focus to higher magnification and CLOSER... pretty much the opposite of what you want. The increased magnification would allow you to back off a little, but the extension actually physically makes the lens longer, sort of nullifying anything you gain with increase mag.
To get greater working distance you need longer focal length.
There are several ways you can accomplish that:
Obviously, one would be to get a different lens.... a macro lens in the 90mm, 100mm or 105mm focal length would do the trick and give you greater working distance. (There are even longer 150mm, 180mm and 200mm macro lenses, but they are more difficult to hand hold, more likely to require smaller apertures in search of adequate depth of field, and thus slower shutter speeds and a tripod. At the highest magnifications, image stabilization has almost no effect.)
Another possibility would be to use a teleconverter between your 40mm lens and the camera. A teleconverter actually changes focal length, but there is some loss of light. A relatively mild 1.4X would make the lens an effective 56mm f/4 (one stop of light loss). Actually a teleconverter doesn't change the minimum focus distance of a lens... However because it does increase magnification (40% greater, with a 1.4X), for same level of magnification you would have increased working distance. A problem with adding a teleconverter is that the additional optics also cost some image quality.... how much or how little is really hard to say. That depends upon the specific lens and teleconverter combination. You'd have to give it a try or find someone who has used a particular 1.4X on the same lens you have.
A third possibility would be to add a diopter/close-up lens/filter to the front of your lens. This would increase it's magnification without changing focal length or significant light loss. But there is likely to be some significant loss of image quality. How much is hard to say... There are wide variety of qualities of diopter/close-up lenses. The cheapies will pretty much ruin image quality. Better, multi-element "diopters" are pricier, but would do less harm to IQ... though there still would be some loss. And, much like using extension tubes, some of what working distance you gain with higher magnification would be lost again to added lens length, with the diopter fitted to the front of it. (Probably not as much loss of working distance as with extension tubes, though.)
A few "bug" examples with different focal lengths:
90mm Tamron macro lens on APS-C crop sensor DSLR (left) and film/full frame (right)...
100mm Canon macro lens on APS-C crop sensor DSLR...
180mm Canon macro lens on film/full frame...
Note the super shallow depth of field in the lefthand image above, which is close to 1:1 magnification. The righthand image of the tarantula is much lower magnification.
And, as a point of comparison, 70-200mm non-macro lens with 25mm extension tube (on film/full frame)
Hope this helps!